This invention relates generally to power control switching arrangements and particularly to switching arrangements for controlling power to heating units in electric ranges and cooktops in response to trigger signals from an electronic controller.
In electronically controlled ranges and cooktops of the type disclosed in commonly assigned U.S. Pat. No. 4,493,980 the electro-mechanical temperature sensing devices are replaced with electronic temperature sensing and control arrangements. In such arrangements an electronic controller, typically a custom programmed microprocessor, monitors the temperature information from a thermistor type sensor in the surface unit. Power switching is performed by a power control triac serially connecting the surface unit heating element to the household AC power supply. The electronic controller generates trigger signals for the triac as appropriate to achieve and maintain the desired temperature condition.
Triacs for power switching are used at least in part because power switching relays are also costly and have relatively limited longevity due to arcing problems in the contacts. However, the power control triac must carry high currents under normal operating conditions requiring large, bulky and costly heat sink structure to dissipate the heat generated in the triac. In one approach considered to relieve the large heat sink requirement, a relatively inexpensive relay rated only for conduction rather than power switching is used to shunt the power control triac during the ON cycles. The relay is closed, shunting the triac after the triac is triggered into conduction, thereby limiting the voltage developed across the relay during closure to a level insufficient to support arcing across the contacts. Once the relay is closed and conducting, the triac's trigger signal is removed and the triac remains non-conductive. To terminate the ON period, the triac is once again triggered into conduction, then the relay is opened and the triac trigger signal is removed. By limiting the conduction period of the triac to relative short times, this arrangement eliminates the need for a large heat sink for the triac. However, it also has certain drawbacks. To be cost effective, a relatively inexpensive relay must be used. Such relays tend to be noisy and have a relatively short useful life. Typically a cost effective relay would have a life expectancy of approximately one million switching cycles. For the type of control environment envisioned for an electronic control package for a cooktoop or range a life expectancy on the order of 4-5 million switching cycles is the goal.
Therefore, there is a need for a power control switching arrangement for use in electronic controls for cooktop surface units and particularly automatic surface units which is more cost effective than the triac/heat sink combination and which overcomes the aforementioned shortcomings of the relay/triac combination.
It is therefore a primary object of the present invention to provide a power control switching arrangement which retains the noise and longevity advantages of semiconductor switching, while eliminating the need for a large heat sink for the power control triac.
It is a further object of the present invention to provide a power control arrangement of the aforementioned type which provides for a switching cycle lifetime greater than that of a low cost conductive relay, quieter operation than a relay circuit, and which may be assembled in a smaller package.